1. Field of the Invention
This invention relates to a flame retardant polyurethane-modified polyisocyanurate. More particularly, this invention relates to the use of a normally liquid, flame retardant polyol ester in a polyurethane-modified polyisocyanurate composition.
2. Description of the Prior Art
One of the efforts which the rigid urethane foam industry has capitalized on in recent years has been the development of a rigid foam which has flame retardance and generates less smoke when it does burn. The rigid foam which has gained the greatest attention and commercial acceptance is polyisocyanurate foam.
Polyisocyanurate refers to isocyanate polymer trimer groups formed by the reaction between isocyanate groups. Although chemically related to polyurethanes, the products known commercially as polyisocyanurates, isocyanurates, or trimer foams differ from ordinary polyurethanes in that the polymer backbone contains the more thermally stable isocyanurate ring structure, in addition to whatever other chemical linkages may be present. The isocyanurate ring structure imparts a greater degree of thermal stability and lower combustibility characteristics to the finished product than those ordinarily obtainable in polyurethanes. The polyisocyanurate linkage is obtained by trimerization of isocyanate groups in the presence of a trimerization catalyst: ##STR1##
The reactants may also contain blowing agents. These foams have excellent resistance to deformation even at temperatures of up to 250.degree. C. and much more prolonged resistance to fire than conventional polyurethane foams. Polyisocyanurate foams are essentially closed-cell with a nominal density of two to ten pounds per cubic foot.
Polyisocyanurate foams are characterized by the absence of labile hydrogens in the polymer structure and by the highly cyclic structure. These characteristics provide the resulting foams with high thermal stability and flame retardance. However, these polymers, having a very high cross-link density, are too friable to be of practical use. Considerable effort has been directed toward modifying these foams to make them acceptable for commercial application. Often times this modification adversely affects the thermal stability and the flammability of these foams. The properties of modified and unmodified foams are also generally adversely affected by flame retardant additives.
One method of modifying polyisocyanurate foams is by adding an elasticizing component. By adding a polyol as an elasticizing agent to the initial reactants then a polyurethane-modified polyisocyanurate foam results. These are sometimes referred to as a urethane-modified isocyanurate foam or a polyurethane-polyisocyanurate hybrid foam.
Polyisocyanurate foams contained only isocyanurate units. There are no polyol components present in the foam. These foams tend to be highly friable and generally require little or no additional flame retardant components. Polyurethane-modified polyisocyanurate foams, on the other hand, contain a polyol component.
These polyurethane-modified polyisocyanurate foams often times have an isocyanate index of 160-200, but isocyanate indexes up to 600 or even 1,000 are possible. These polyurethane-modified isocyanurate foams have improved friability at the cost of decreased thermal stability and increased flammability. Thus, a flame retardant component is generally required.
Copending application U.S. Ser. No. 385,782, filed June 7, 1982 claims a normally liquid flame retardant polyol ester prepared by a process comprising reacting a halogen containing anhydride of a dicarboxylic acid or a mixture of said anhydride with an aliphatic polyol having at least three hydroxy groups and an aliphatic diol, to obtain a substantially fully esterified halogen containing product in which the acid number does not exceed 10 with the optional removal of unreacted diol during the course of the reacting or subsequent to the reacting. The subject invention claims the use of this flame retardant polyol composition in polyurethane-modified polyisocyanurate foams.